Ankle-foot orthotics (AFOs) are external orthopaedic devices that provide support for foot-drop or ankle instability associated with many neuromuscular disorders. Currently, most AFOs are articulated (freely moving) or rigid (maximal resistance). Dynamic joint stiffness (JS) can be calculated as the slope of a joint angle vs. moment plot and is an important constraint on the motor control system (Hunter & Kearney, 1982), so understanding JS is essential to orthotic design and evaluation of neuromuscular diseases (Davis & DeLuca, 1995). JS has not been examined experimentally with the use of AFOs and the amount of time required to adapt to changes in AFO resistance is unknown. Quantifying the effects of variable AFO resistance on joint mechanics and electromyographic (EMG) signals during overground and treadmill walking and step-downs was the focus of this research.

Bilateral AFOs were instrumented with electro-hydraulic disc-brakes, which allowed adjustable AFO resistance. The study design was prospective, randomized, and comprised 12 healthy adult males. Three-dimensional kinematics and kinetics and EMG data were collected. The instrumented AFOs allowed for adaptation time to be quantified and for rapid changes in AFO condition. Time-frequency analysis, using wavelets, was used for EMG analysis, and antagonist muscular co-activation was quantified.

There was an increase in EMG amplitude for most muscles with AFO use, and EMG amplitude increased with greater resistance. Time and frequency-dependent changes occurred in EMG data, with a trend toward greater co-activation with AFO use. No significant differences in resultant ankle or knee JS existed, independent of AFO use or resistance. Significant differences occurred in mechanical joint energetics with AFO use. Adaptation to changes in AFO condition happened within one stride of a condition change.

Despite AFO use and level of resistance, resultant ankle and knee JS remained unchanged for walking and step-downs, but there were significant changes in EMG and mechanical joint energetics. Significant differences in muscle activity were discovered between treadmill vs. overground walking. Overall, results were consistent with JS and motor control being a function of task (Stefanyshyn & Nigg, 1998), and, therefore, task-specific AFO resistance may be more desirable in clinical populations.